Pregnancy Due Date Calculator
Due date by Naegele's rule, plus gestational age and trimester for any date.
Quick answer
- Formula: Due date = LMP + 280 days (Naegele's rule) · Gestational age = today − LMP
- Worked example — Estimated due date: October 8, 2026 (full example below).
Pregnancy Due Date Calculator
| Estimated due date | October 8, 2026 |
| Gestational age on the 'today' date | 12 weeks, 0 days |
| Trimester | First trimester |
An LMP of January 1, 2026 gives a due date of October 8, 2026. On March 26, 2026 the pregnancy is 12 weeks, 0 days along — first trimester.
Naegele's rule estimates the due date as 280 days (40 weeks) from the first day of the last menstrual period (LMP) — the standard method used in obstetrics since the 19th century, equivalent to adding a year, subtracting three months and adding seven days. Enter today's date as well to see the current gestational age and trimester.
Only about 1 in 20 babies arrive on the exact due date — it marks the centre of a normal window, and early ultrasound dating can revise it. This tool is educational and is not medical advice.
The formula
Worked example
An LMP of January 1, 2026 gives a due date of October 8, 2026. On March 26, 2026 the pregnancy is 12 weeks, 0 days along — first trimester.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
How are the trimesters divided?
First trimester: up to 13 weeks 6 days. Second: 14 weeks to 27 weeks 6 days. Third: 28 weeks until birth. This calculator rounds those boundaries to week 14 and week 28.
What if my cycles are longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Adjust the LMP by the difference — for a 32-day cycle, use an LMP four days later — or rely on an early ultrasound date.
Why might my clinician change my due date?
A first-trimester ultrasound measures the embryo directly and is more precise than LMP recall, so clinicians commonly revise the date if the two disagree by more than about a week.
Category: Health & Fitness calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
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